How do I write a function that takes both owned and non-owned string collections?

Question

I\'m having trouble writing a function that takes a collection of strings as parameter. My function looks like this:

type StrList<\'a> = Vec<&\'         


        

Answer 1

Although String and &str are very closely related, they are not identical. Here's what your vectors look like in memory:

v1 ---> [ { 0x7890, // pointer to "a" + 7 unused bytes
            1 }     // length of "a"
          { 0x7898, // pointer to "b" + 7 unused bytes
            1 } ]   // length

v2 ---> [ { 0x1230 // pointer to "a" + 7 unused bytes (a different copy)
            8      // capacity
            1 }    // length
          { 0x1238 // pointer ...
            8      // capacity
            1 } ]  // length

Here each line is the same amount of memory (four or eight bytes depending on pointer size). You can't take the memory of one of these and treat it like the other. The memory layout doesn't match up. The items are of different sized and have different layout. For example, if v1 stores its items starting at address X and v2 stores its items starting at address Y, then v1[1] is at address X + 8 but v2[1] is at address Y + 12.

What you can do is write a generic function like this:

fn my_func>(list: &[T]) {
    for s in list {
        println!("{}", s.as_ref());
    }
}

Then the compiler can generate appropriate code for both &[String] and &[&str] as well as other types if they implement AsRef.